How to adjust/shift a hour for graph

[baldur]

I make a script to detect a value, example 11am ~ 12am, and report at 12am, cacti take
this value at 12am and make a graph, but the time become to 12am ~ 13pm.
(Attach : cacti.png )
Do anyone know how to offset the graph time a hour ?
(Attach : cacti2.png )